Question: On an algebra test, there were $5x$ problems. Lucky Lacy missed $x$ of them. What percent of the problems did she get correct?
Solution: If Lucky Lacy missed $x$ out of $5x$ problems, then she must have gotten $5x-x=4x$ of them correct. Therefore, the percentage of problems that Lacy got correct is  $\frac{4x}{5x}=\frac{4}{5}=\frac{80}{100}=\boxed{80 \%}$.